1. Field of the Invention
The present invention pertains to a radio communication system, a radio device, a communication control method, and a communication control program in a computer-readable storage medium, applicable for radio communication using a plurality of communication channels between, for example, a radio base station and a radio communication terminal.
2. Description of Related Art
In communication channel allocations in a 2.4 GHz band wireless LAN (Local Area Network) of IEEE 802.11 method, for example, channels of approximately 20 MHz-bandwidth are arranged with intervals of 5 MHz so adjacent channels interfere with each other. Thus, only four channels (ch1, ch6, ch11, ch14) at a maximum are available in Japan, as channels that will not interfere (overlap) with each other. This is why a base station needs to be designed in a view of the four channels that will not interfere with each other within an area.
In such a radio communication system, communication channel switch will be performed, for example, when radio interference from other radio communication systems or radar such as weather radar in similar frequencies is detected.
In systems of the related art, a frequency channel switching device determines whether to switch a frequency channel based on dynamically-changing line status between corresponding wireless LAN devices and the wireless LAN devices switch to the determined frequency channel (for example, see Japanese Patent Application Laid-Open No. 2002-271336 (document 1)).
Next, problems of those radio communication systems related to the present invention will be described with reference to FIG. 1. FIG. 1 shows a channel switch procedure by a radio base station in related art by taking a control frame of IEEE 802.11 wireless LAN as an example. In FIG. 1, the dotted lines represent broadcast frames and the continuous lines represent unicast frames. The each frame uses a radio channel shown in the parentheses respectively and beacon frames and ACK (Acknowledge) frames are not shown.
When a communication channel is to be switched from ch1 to ch6 in the radio communication system shown in FIG. 1, a radio base station firstly releases its attribution to ch1 and switches the communication channel to ch6 (step S51).
The radio communication terminal disconnects the connection when releasing its attribution and starts a searching operation to find a new destination to attribute. Since the radio base station operates by ch6, the radio base station returns a probe response to a probe request of ch6 (step S52) to perform attribution process (step S53). With this, data are transmitted and received by ch6 (step S54).
As described above, in the attribution searching operation, an attribution is released once and searches a new attribution, so there has been a problem that the radio communication terminal may belong to a different radio base station after a communication channel switch.
Further, since the radio communication terminal disconnects the connection when releasing its attribution, communication cannot be operated after the attribution release and until the start of data transmission and reception by ch6.
Also, the art disclosed in document 1 is a technique for determining whether or not to switch a channel based on the line state and it is not made in view of preventing disconnection during a channel switch operation or maintaining a connection between the connected radio devices.